Multilayer anode and lithium secondary battery including the same

ABSTRACT

A multilayer anode includes an anode collector, and a plurality of anode mixture layers sequentially stacked on at least one surface of the anode collector, and including natural graphite as an anode active material. A weight ratio of the natural graphite in innermost and outermost anode mixture layers is greater than a weight ratio of the natural graphite in an anode mixture layer located between the innermost and outermost anode mixture layers, in a stacking direction of the plurality of anode mixture layers. Performance of a cell may be improved and calendering-calender contamination occurring in a calendering process and an electrode stripping phenomenon may be prevented.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims the benefit under 35 USC 119(a) of Korean PatentApplication No. 10-2018-0110336 filed on Sep. 14, 2018 in the KoreanIntellectual Property Office, the entire disclosure of which isincorporated herein by reference for all purposes.

BACKGROUND

The present disclosure relates to an anode including a multilayer anodemixture layer, and more particularly, to a multilayer anode withmultiple layers having different contents of natural graphite containedas an anode active material in an anode mixture layer, and a lithiumsecondary battery including the same.

As technological development and demand for mobile devices haveincreased, there has been a rapid increase in demand for secondarybatteries as energy sources. Among such secondary batteries, lithiumsecondary batteries, exhibiting high energy density and operatingpotential and which have a long cycle life and a low self-dischargerate, have been commercialized and widely used.

As interest in environmental issues has increased in recent years,research into electric vehicles (EVs) and hybrid electric vehicles(HEVs), which may replace fossil-fueled vehicles such as gasolinevehicles, diesel vehicles and the like, significant causes of airpollution, has been conducted. Lithium secondary batteries having highenergy density, high discharge voltage and output stability have mainlybeen researched and used as power sources for electric vehicles (EVs)and hybrid electric vehicles (HEVs).

Such a lithium secondary battery is under development as a model capableof implementing a high voltage and a high capacity in line with consumerdemand. To implement a high capacity thereof, a lithium secondarybattery is required to have an optimization process of a cathodematerial, an anode material, a separator, and an electrolyte, fourelements of the lithium secondary battery, within a limited space.

On the other hand, the use of artificial graphite in the place ofnatural graphite as an anode active material has been increasing inrecent years in order to improve cell lifespan and the like in theproduction of an anode used in a secondary battery. However, as theartificial graphite content increases, the adhesive force between acurrent collector and an electrode is lowered, and a phenomenon in whichthe electrode is separated from a substrate in the electrolyte may beserious.

Further, in the process of calendering an electrode using artificialgraphite, an active material on the surface of an electrode mixturelayer is deposited on a calendering calender, and thus, calendercontamination becomes serious, which causes a problem in whichproductivity in a calendering process is significantly lowered.

SUMMARY

An aspect of the present disclosure is to provide a multilayer anode inwhich calendering-calender contamination occurring in a calenderingprocess and an electrode stripping phenomenon may be prevented, and alithium secondary battery including the same.

According to an embodiment of the present disclosure, a multilayer anodeincludes an anode collector, and a plurality of anode mixture layerssequentially stacked on at least one surface of the anode collector andincluding natural graphite as an anode active material. A weight ratioof the natural graphite in innermost and outermost anode mixture layersis greater than a weight ratio of the natural graphite in an anodemixture layer located between the innermost and outermost anode mixturelayers in a stacking direction, from among the plurality of anodemixture layers.

The anode mixture layer may be comprised of three or more layers.

A weight ratio of the natural graphite in portions of the plurality ofanode mixture layers, corresponding to 0 to 20% and 80 to 100% of atotal height of the plurality of anode mixture layers, from the anodecollector, in a thickness direction of the plurality of anode mixturelayers, may be greater than a weight ratio in a portion of the pluralityof anode mixture layers, except for the portions thereof.

The natural graphite of the innermost anode mixture layer may becontained in an amount of 50 to 100 wt % based on a total activematerial weight of the innermost anode mixture layer.

The natural graphite of the outermost anode mixture layer may becontained in an amount of 50 to 100 wt % based on a total activematerial weight of the outermost anode mixture layer.

The anode mixture layer may further include one or more anode activematerials selected from artificial graphite, natural graphite, softcarbon, hard carbon, acetylene carbon black, Ketjen black, carbonnanotubes, carbon nanofiber and silicon oxide.

The plurality of anode mixture layers may further include a binder and aconductive material.

According to an embodiment of the present disclosure, a lithiumsecondary battery including the multilayer anode as described above isprovided.

BRIEF DESCRIPTION OF DRAWINGS

The above and other aspects, features, and advantages of the presentdisclosure will be more clearly understood from the following detaileddescription, taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 schematically illustrates a multilayer anode according to anembodiment of the present disclosure;

FIG. 2 schematically illustrates a multilayer anode according to anotherembodiment of the present disclosure;

FIG. 3 is an image showing a calendering calender after manufacturing ananode according to Embodiment Example 2 of the present disclosure;

FIG. 4 is an image showing a calendering calender after manufacturing ananode according to Comparative Example 1 of the present disclosure; and

FIG. 5A is an image in Comparative Example 1 and FIG. 5B is an image inEmbodiment Example 2 of the present disclosure, respectivelyillustrating the degree of anode stripping after impregnation of ananode with an electrolyte, for comparison therebetween.

DETAILED DESCRIPTION

The following detailed description is provided to assist the reader ingaining a comprehensive understanding of the methods, apparatuses,and/or systems described herein. However, various changes,modifications, and equivalents of the methods, apparatuses, and/orsystems described herein will be apparent to one of ordinary skill inthe art. The sequences of operations described herein are merelyexamples, and are not limited to those set forth herein, but may bechanged, as will be apparent to one of ordinary skill in the art, withthe exception of operations necessarily occurring in a certain order.Also, descriptions of functions and constructions that would be wellknown to one of ordinary skill in the art may be omitted for increasedclarity and conciseness.

The terminology used herein describes particular embodiments only, andthe present disclosure is not limited thereby. As used herein, thesingular forms “a,” “an,” and “the” are intended to include the pluralforms as well, unless the context clearly indicates otherwise. It willbe further understood that the terms “including”, “comprises,” and/or“comprising” when used in this specification, specify the presence ofstated features, integers, steps, operations, members, elements, and/orgroups thereof, but do not preclude the presence or addition of one ormore other features, integers, steps, operations, members, elements,and/or groups thereof.

Throughout the specification, it will be understood that when anelement, such as a layer, region or wafer (substrate), is referred to asbeing “on,” “connected to,” or “coupled to” another element, it may bedirectly “on,” “connected to,” or “coupled to” the other element orother elements intervening therebetween may be present. In contrast,when an element is referred to as being “directly on,” “directlyconnected to,” or “directly coupled to” another element, there may be noelements or layers intervening therebetween. Like numerals refer to likeelements throughout. As used herein, the term “and/or” includes any andall combinations of one or more of the associated listed items.

The drawings may not be to scale, and the relative size, proportions,and depiction of elements in the drawings may be exaggerated forclarity, illustration, and convenience.

Hereinafter, embodiments of the present disclosure will be describedwith reference to various embodiments. However, the embodiments of thepresent disclosure can be modified into various other forms, and thescope of the present disclosure is not limited to the embodimentsdescribed below.

A multilayer anode and a lithium secondary battery including the sameare provided. The secondary battery is formed by forming an electrodeassembly by interposing a separator between a cathode and an anode, tobe wound or stacked, and by storing the electrode assembly inside a caseand then filling the case with an electrolyte. The electrode, forexample, the cathode and the anode of the secondary battery are formedby applying a cathode active material or an anode active material on acurrent collector and calendering the same. For example, in performing acalendering process of the electrode, the electrode coated with anelectrode active material is passed between an upper calender and alower calender.

In such a secondary battery calendering apparatus, an active materialand a binder on the surface of the electrode stain the calenderingcalender in a high-pressure calendering process, thereby causing aproblem of calender contamination. In a case in which the calender iscontaminated, an active material aggregate present in a mixture layermoves to the calender, causing cyclic sticking problems. Therefore, in acase in which the calender is contaminated, it is necessary to clean thecalender. Therefore, productivity may be lowered due to an increase inprocess time and cost.

The use of artificial graphite as an anode active material instead ofnatural graphite has been increasing in recent years, but as theartificial graphite content in the electrode is increased, the adhesionbetween a current collector and an electrode is lowered, and thus, theelectrode may be separated from a substrate in an impregnation state ofan electrolyte.

A multilayer anode according to an embodiment includes an anodecollector 10, and a plurality of anode mixture layers sequentiallystacked on at least one surface of the anode collector 10, and includingnatural graphite as an anode active material. In this case, a weightratio of the natural graphite in innermost and outermost anode mixturelayers is greater than a weight ratio of the natural graphite in ananode mixture layer located between the innermost and outermost anodemixture layers, in a stacking direction of the plurality of anodemixture layers.

The anode mixture layers may be comprised of at least three layers. Forexample, the anode mixture layers may be comprised of a first anodemixture layer 20 formed on an innermost side to be in contact with thecurrent collector in the stacking direction of the anode mixture layers,a second anode mixture layer 30 formed on the first anode mixture layer20, and a third anode mixture layer 40 disposed on the second anodemixture layer 30 and disposed on an outermost side in the stackingdirection of the anode mixture layers. If necessary, a fourth anodemixture layer 50 may be formed on the third anode mixture layer 40. Inthis case, the anode mixture layer in which the fourth anode mixturelayer 50 is an outermost anode mixture layer may be provided.

The number of the anode mixture layers is not particularly limited, butin detail, may be 3 to 4 layers. If the number of the anode mixturelayers exceeds 4 layers, a process cost increases due to an additionalprocess, and furthermore, a manufacturing yield decreases due to arepeated application process.

To prevent the above-described calender contamination and electrodestripping phenomenon, and to secure output and lifespan characteristicsof the battery, a weight ratio of the natural graphite in portions ofthe plurality of anode mixture layers, corresponding to 0 to 20% and 80to 100% of a total height of the plurality of anode mixture layers, fromthe anode collector, in a thickness direction of the anode mixturelayers, may be greater than a weight ratio of the natural graphite in aportion of the plurality of anode mixture layers, except for theportions thereof. For example, when a region of 0 to 20% of the totalheight of the plurality of anode mixture layers is defined as A, aregion of 21 to 79% thereof is defined as B, and a region of 80 to 100%is defined as C, the ratios of the contents of natural graphitecontained in respective regions may be, in detail, A/B>1 and C/B>1.

As described later, the anode mixture layer according to an embodimentmay further include an anode active material such as artificialgraphite, soft carbon, hard carbon, acetylene carbon black, Ketjenblack, carbon nanotubes, carbon nanofiber, silicon oxide and the like,in addition to natural graphite. In this case, a relatively large amountof natural graphite is contained in a range of 0 to 20% of the totalheight of the plurality of anode mixture layers, for example, in aportion thereof contacting the anode collector 10, thereby improving theadhesion between the collector and the electrode and preventing anelectrode stripping phenomenon. Further, a relatively large amount ofnatural graphite is contained in a range of 80% to 100% of the totalheight of the plurality of anode mixture layers, for example, in aportion thereof in which the calender and the anode mixture layer are incontact with each other, thereby reducing calender contamination. Inaddition, the content of an active material such as artificial graphiteis increased in the remaining portion, thereby ensuring lifespancharacteristics and output characteristics of the entire battery.

For example, the number of the anode mixture layers may be three orfour. In this case, the multilayer anode according to an embodiment maybe configured in such a manner that an amount of the natural graphitecontained in the innermost and outermost anode mixture layers in thestacking direction of the anode mixture layers is more than an amount ofnatural graphite contained in the entire anode mixture layer.

As described above, according to an embodiment, the occurrence of aproblem of a manufacturing process may be prevented withoutsignificantly decreasing the ratio of the total artificial graphiteamount by increasing the content of the natural graphite only in aregion in which anode stripping and calendering calender contaminationproblems occur in the electrode mixture layers. In more detail, sinceparticle deformation of natural graphite is relatively facilitated ascompared with that of artificial graphite and spring-back forcegenerated in the calendering process is thus relatively low, shearstress occurring between a rotating calendering calender and the anodemixture layers is reduced. as the shear stress decreases, separation ofan inner layer of graphite is difficult. As a result, by increasing theoutermost natural graphite content, calender contamination issignificantly reduced even after high-pressure calendering. Further, inthe case of natural graphite having the same particle size as that ofartificial graphite, since the surface area of the natural graphite issmaller than that of a modular artificial graphite, binding forcethereof with the current collector is higher than that of the artificialgraphite even using the same binder content. As a result, a phenomenonin which the collector and the electrode mixture layers are separatedfrom each other due to penetration of an electrolyte may be prevented byincreasing the content of natural graphite in an innermost anode mixturelayer.

As a result, according to an embodiment of the present disclosure,occurrence of an anode stripping phenomenon and calendering calendercontamination may be prevented while improving cell performance.

On the other hand, the natural graphite of the innermost anode mixturelayer may be contained in an amount of 50 to 100 wt %, in more detail,70 to 90 wt %, based on the total active material weight of theinnermost anode mixture layer. If the content of natural graphite is 50wt % or less, the adhesive force between the anode mixture layer and thecurrent collector is not sufficient, and an electrode strippingphenomenon may occur after impregnation with the electrolyte.

The natural graphite of an outermost anode mixture layer may becontained in an amount of 50 to 100 wt %, in more detail, 70 to 90 wt %,based on the total weight of an active material of the outermost anodemixture layer. If the content of natural graphite is 50 wt % or less, itis difficult to reduce the problem of calender contamination byartificial graphite.

In an embodiment, the anode mixture layer may further include one ormore anode active materials in addition to natural graphite as an anodeactive material. The kind of the anode active material is notparticularly limited. For example, artificial graphite, soft carbon,hard carbon, acetylene carbon black, Ketjen black, carbon nanotubes,carbon nanofiber, silicon oxide and the like may be used.

The anode mixture layer may further include a binder and a conductivematerial, as required. The binder is a component that assists in bondingbetween the active material and a conductive material and bonding to acurrent collector. A material used as the binder is not particularlylimited, but polyvinylidene fluoride (PVDF), polyvinyl alcohol,Carboxymethylcellulose (CMC), starch, hydroxypropylcellulose,regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene,polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM),sulfonated EPDM, styrene-butylene rubber (SBR), fluorine rubber, or amussel protein, a polyacrylate-based binder, a polyolefin-based binder,a silane-based binder and the like may be used, and in detail, one ormore selected from PVDF, SBR, a mussel protein, a polyolefin basedbinder, a polyacrylate-based binder, and a silane-based binder may beused. In addition, the kinds of the binders used in the anode mixturelayers may be the same as each other or may be different from eachother.

The conductive material is not particularly limited as long as it is asubstance included for improving electronic conductivity and hasconductivity without causing chemical change in the battery. Forexample, as the conductive material, graphite such as natural graphiteor artificial graphite; carbon black such as carbon black, acetyleneblack, Ketjen black, channel black, furnace black, lamp black, thermalblack or the like; a conductive fiber such as carbon fiber and metalfiber; a metal powder such as carbon fluoride powder, aluminum powder,nickel powder or the like, a conductive whisker such as zinc oxide,potassium titanate or the like, a conductive metal oxide such astitanium oxide or the like, a conductive material such as polyphenylenederivatives, carbon nanotubes, grapheme or the like, and the like may beused. The types of the conductive materials used in the respective anodemixture layers may be the same or different from each other.

A method of manufacturing a multilayer anode according to an embodimentis not particularly limited, and an anode active material slurry may beprepared according to a known method, and a method of coating an anodecollector with the active materials may also be performed using a knownmethod.

According to another embodiment, a lithium secondary battery includingthe above-described multilayer electrode may be provided, and an anodestripping phenomenon may be prevented while improving performance of acell.

EMBODIMENT EXAMPLES

Hereinafter, the present disclosure will be described in more detailwith reference to example embodiments. The following examples areintended to further illustrate the present disclosure and are notintended to limit the present disclosure.

Embodiment Example 1

As a binder, carboxymethyl cellulose (CMC) and styrene-butadiene rubber(SBR) were mixed in an amount of 1.0 wt % and 2.0 wt %, respectively,and natural graphite and artificial graphite were mixed in an amount of67.9 wt % and 29.1 wt %, respectively, as an anode active material.Distilled water was added such that an ultimate solid weight was about52% to be mixed for 150 minutes to prepare first and third layer anodeslurries.

Carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR) as abinder, and artificial graphite as an anode active material, were mixedin an amount of 1.0 wt %, 2.0 wt %, and 97 wt %, respectively. Then,distilled water was added such that an ultimate solid weight was about52%, to be mixed for 150 minutes to prepare second layer anode slurry.

The first layer anode slurry was applied to one side of a copper foil(having a thickness of 6 μm) to a thickness of 32 μm and was thenapplied to the opposite side to have the same thickness, followed bydrying. The second layer anode slurry was applied to one side of a firstanode mixture layer dried as described above to a thickness of 86 μm andwas then applied to the opposite side thereof to have the samethickness, followed by drying. Ultimately, the third layer anode slurrywas applied to one side of a second anode mixture layer dried asdescribed above to a thickness of 32 μm and was then applied to theopposite side thereof to have the same thickness, followed by drying.

Thereafter, about 1,000 m of the dried anode mixture layer wascalendered by using the apparatus illustrated in FIG. 3 to prepare ananode having an electrode thickness of 90 μm on one surface thereof.

Embodiment Example 2

Carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR) weremixed as the binder in an amount of 1.0 wt % and 2.0 wt %, respectively,and natural graphite and artificial graphite were mixed as an anodeactive material in an amount of 87.3 wt % and 9.7 wt %, respectively,and distilled water was then added thereto such that an ultimate solidweight was about 524 to be mixed for 150 minutes to prepare first andthird layer anode slurries.

Carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR) as abinder, and artificial graphite as an anode active material, were mixedin an amount of 1.0 wt %, 2.0 wt %, and 97 wt %, respectively. Then,distilled water was added such that an ultimate solid weight was about52%, to be mixed for 150 minutes to prepare second layer anode slurry.

The first layer anode slurry was applied to one side of a copper foil(having a thickness of 6 μm) to a thickness of 25 μm and was thenapplied to the opposite side thereof to have the same thickness,followed by drying. The second layer anode slurry was applied to oneside of a first anode mixture layer dried as described above to athickness of 99 μm and was then applied to the opposite side thereof tohave the same thickness, followed by drying. Ultimately, the third layeranode slurry was applied to one side of a second anode mixture layerdried as described above to a thickness of 25 μm and was then applied tothe opposite side thereof to have the same thickness, followed bydrying.

Thereafter, about 1,000 m of the dried anode mixture layer wascalendered by using the apparatus illustrated in FIG. 3 to prepare ananode having an electrode thickness of 90 μm on one surface thereof.

FIG. 3 illustrates a calender used for calendering the anode. Referringto FIG. 4 showing a calender used for calendering an anode ofComparative Example 1 to be described later, it can be confirmed that acalender surface is stained with an extremely small amount of an active.

Comparative Example 1

Carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR) weremixed as a binder in an amount of 1.0 wt % and 2.0 wt %, respectively,and natural graphite and artificial graphite were mixed as an anodeactive material in an amount of 9.7 wt % and 87.3 wt %, respectively,and distilled water was then added thereto such that an ultimate solidweight was about 52%, to be mixed for 150 minutes to prepare an anodeslurry.

The anode slurry was applied to one side of a copper foil (having athickness of 6 μm) to a thickness of 150 μm and was then applied to theopposite side thereof to have the same thickness, followed by drying.

Thereafter, about 1,000 m of an anode mixture layer dried as describedabove was calendered by using the apparatus illustrated in FIG. 3, toprepare an anode having an electrode thickness of 90 μm on one surfacethereof.

FIG. 4 illustrates a calender used for calendering the anode. Asdescribed above, it can be confirmed that the active material mass movesto the calender during a calendering process due to seriouscontamination of the calender, and the calendered anode mixture layer isstruck.

Comparative Example 2

Carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR) weremixed as a binder in an amount of 1.0 wt % and 2.0 wt %, respectively,and natural graphite and artificial graphite were mixed as an anodeactive material in an amount of 29.1 wt % and 67.9 wt %, respectively,and distilled water was then added thereto such that an ultimate solidweight was about 52%, to be mixed for 150 minutes to prepare an anodeslurry.

The anode slurry was applied to one side of a copper foil (having athickness of 6 μm) to a thickness of 150 μm and was then applied to theopposite side thereof to have the same thickness, followed by drying.

Thereafter, about 1,000 m of an anode mixture layer dried as describedabove was calendered by using the apparatus illustrated in FIG. 3, toprepare an anode having an electrode thickness of 90 μm on one surfacethereof.

Comparative Example 3

Carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR) weremixed as a binder in an amount of 1.0 wt % and 2.0 wt %, respectively,and natural graphite and artificial graphite were mixed as an anodeactive material in an amount of 48.5 wt % and 48.5 wt %, respectively,and distilled water was then added thereto such that an ultimate solidweight was about 52%, to be mixed for 150 minutes to prepare an anodeslurry.

The anode slurry was applied to one side of a copper foil (having athickness of 6 pun) to a thickness of 150 μm and was then applied to theopposite side thereof to have the same thickness, followed by drying.

Thereafter, about 1,000 m of an anode mixture layer dried as describedabove was calendered by using the apparatus illustrated in FIG. 3, toprepare an anode having an electrode thickness of 90 μm on one surfacethereof.

Comparative Example 4

Carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR) weremixed as a binder in an amount of 1.0 wt % and 2.0 wt %, respectively,and natural graphite and artificial graphite were mixed as an anodeactive material in an amount of 67.9 wt % and 29.1 wt %, respectively,and distilled water was then added thereto such that an ultimate solidweight was about 52%, to be mixed for 150 minutes to prepare a firstlayer anode slurry.

Carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR) weremixed as a binder in an amount of 1.0 wt % and 2.0 wt %, respectively,and natural graphite and artificial graphite were mixed as an anodeactive material in an amount of 9.7 wt % and 87.3 wt %, respectively,and distilled water was then added thereto such that an ultimate solidweight was about 52%, to be mixed for 150 minutes to prepare a secondlayer anode slurry.

The first layer anode slurry was applied to one side of a copper foil(having a thickness of 6 μm) to a thickness of 50 μm and was thenapplied to the opposite side thereof to have the same thickness,followed by drying. The second layer anode slurry was applied to oneside of a first anode mixture layer dried as described above to athickness of 100 μm and was then applied to the opposite side thereof tohave the same thickness, followed by drying.

Thereafter, about 1,000 m of the dried anode mixture layer wascalendered by using the apparatus illustrated in FIG. 3, to prepare ananode having an electrode thickness of 90 μm on one surface thereof.

Experimental Example

Measurement of Collector Adhesion

To measure adhesive force between a mixture layer and a currentcollector in anodes prepared in Embodiment Examples 1 and 2 andComparative Examples 1 to 4, a 3M tape having a 18 mm width was attachedonto each electrode and a 90 degree peel test was conducted.

The force at the time of separation of the mixture layer and the currentcollector was measured, and the adhesive strength of the currentcollector was calculated by dividing the measured force by the width ofthe tape. The calculated values are illustrated in Table 1.

Measurement of Electrode Stripping during Impregnation with Electrolyte

The electrodes prepared in Embodiment Examples 1 and 2 and ComparativeExamples 1 to 4 were punched into a circle having a diameter of 36 mmand then placed in a plastic bottle filled with 20 ml of an electrolyteto be impregnated in the electrolyte and allowed to stand for 3 hours.

Whether or not the electrolyte penetrated into the mixture layer andwhether or not electrode stripping occurred between the anode mixturelayer and the current collector were visually observed, and presence andabsence of stripping is illustrated in Table 1 as ◯ (occurrence ofstripping) and x (non-occurrence).

Calendering Calender Electrode Point Measurement

Calendering was carried out at a running speed of 20 m/min for theelectrode of 1,000 m prepared in Embodiment Examples 1 and 2 andComparative Examples 1 to 4. Table 1 illustrates calendering runninglengths until active material aggregates are separated from the anodemixture layer due to calender contamination during a calendering processand are attached to the calender, continuously causing damaging on thesurface of the electrode.

0.33 C Charge/Discharge Capacity Retention Rate Measurement

A battery was fabricated using the electrodes prepared in EmbodimentExamples 1 and 2 and Comparative Examples 1 to 4.

Each prepared battery was charged until the voltage reached 4.2V in aconstant current (CC) mode of ⅓C at a temperature of 45° C. Thereafter,after discharging until the voltage reached 2.5 V in the constantcurrent (CC) mode of ⅓C, additional discharging continued until acurrent value was reduced to a level of 0.05% of an initial currentvalue in a constant voltage (CV) mode, and a discharge capacity of afirst order was confirmed.

Thereafter, the same charge and discharge operations were performed fora total of 200 times, and a discharge capacity measured in the lastorder was divided by the discharge capacity of the first order tocalculate the 0.33 C charge/discharge capacity retention rate. Thecalculation results obtained thereby are illustrated in Table 1.

TABLE 1 Embodiment Comparative Example Example Classification 1 2 3 4 12 Total Natural 10 30 50 30 30 30 Graphite Content (weight %) InnermostNatural 10 30 50 70 70 90 Graphite Content (weight %) Outermost Natural10 30 50 10 70 90 Graphite Content (weight %) Current Collector 0.210.23 0.27 0.29 0.30 0.32 Adhesive Force (N/cm) Whether or not ∘ ∘ x x xx electrode stripping occurs at the time of impregnation withelectrolyte Calendering 380m 750m No 430m No No Calender OccurrenceOccurrence Occurrence Electrode Damaging Point in Time (1,000mproduction) 0.33 C 98% 97% 92% 97% 98% 98% Charge/Discharge CapacityRetention Rate (@200cycle, 45° C.)

As can be seen from the results of Table 1 and FIGS. 3 to 5, in the caseof the electrodes of Embodiment Examples 1 and 2, in which the contentsof natural graphite in innermost and outermost anode mixture layers arehigh as compared with the total natural graphite content; the charge anddischarge capacity retention rate was excellent, a stripping phenomenonof the electrode mixture layer did not occur at the time of impregnationwith an electrolyte, and a stain or damaging phenomenon of the electrodedue to calender contamination during a calendering process did notoccur.

Meanwhile, in Comparative Examples 1, 2 and 3 in which the naturalgraphite contents in the anode mixture layer are all the same asillustrated in Table 1, the retention rate of the charge and dischargecapacity increases as the natural graphite content decreases. However,it can be confirmed that electrode stripping occurs during impregnationwith an electrolyte, and the point of time when the electrode is struckdue to calender contamination during a calendering process is reduced.

In the case of Comparative Example 4, the content of natural graphite onan innermost side of the anode mixture layer was high compared to thetotal content of natural graphite, such that the stripping of theelectrode mixture layer did not occur upon impregnation withelectrolyte. However, the natural graphite content in an outermost layerof the anode mixture layer was relatively low, and thus, damaging due tocalender contamination during a calendering process occurred at a pointof about 430 m.

As set forth above, in a multilayer anode and a lithium secondarybattery including the same according to an embodiment, performance of acell may be improved, calendering-calender contamination occurring in acalendering process and an electrode stripping phenomenon may beprevented.

While this disclosure includes specific examples, it will be apparent toone of ordinary skill in the art that various changes in form anddetails may be made in these examples without departing from the spiritand scope of the claims and their equivalents. The examples describedherein are to be considered in a descriptive sense only, and not forpurposes of limitation. Descriptions of features or aspects in eachexample are to be considered as being applicable to similar features oraspects in other examples. Suitable results may be achieved if thedescribed techniques are performed to have a different order, and/or ifcomponents in a described system, architecture, device, or circuit arecombined in a different manner, and/or replaced or supplemented by othercomponents or their equivalents. Therefore, the scope of the disclosureis defined not by the detailed description, but by the claims and theirequivalents, and all variations within the scope of the claims and theirequivalents are to be construed as being included in the disclosure.

What is claimed is:
 1. A multilayer anode comprising: an anodecollector; and a plurality of anode mixture layers sequentially stackedon at least one surface of the anode collector, wherein an innermostanode mixture layer and an outermost anode mixture layer of the anodemixture layers comprise natural graphite as an anode active material,wherein an anode mixture layer located between the innermost anodemixture layer and the outermost anode mixture layer comprises a carbonmaterial as anode active material, wherein among the plurality of anodemixture layers, a weight ratio of the natural graphite in the innermostanode mixture layer over the total active material of the innermostanode mixture layer and a weight ratio of the natural graphite in theoutermost anode mixture layer over the total anode active material ofthe outermost anode mixture layer are each greater than a weight ratioof natural graphite in the anode mixture layer located between theinnermost and outermost anode mixture layers over the total anode activematerial of the anode mixture layer located between the innermost andoutermost anode mixture layers, in a stacking direction.
 2. Themultilayer anode of claim 1, wherein the plurality of anode mixturelayers is comprised of three or more layers.
 3. The multilayer anode ofclaim 1, wherein a total height of the plurality of anode mixture layersin the stacking direction is divided in an innermost region occupied bythe innermost anode mixture layer, an outermost region occupied by theoutermost anode mixture layer and a middle region occupied by the anodemixture layer located between the innermost anode mixture layer and theoutermost anode mixture layer, and wherein the innermost region extendsfrom the anode collector to a first height which is 20% of the totalheight of the plurality of anode mixture layers in the stackingdirection, the middle region extends from the first height to a secondheight of the plurality of anode mixture layers which is 80% of thetotal height and the outermost region extends from the second height toa third height which is 100% of the total height of the plurality ofanode mixture layers.
 4. The multilayer anode of claim 1, wherein thenatural graphite of the innermost anode mixture layer is contained in anamount of 50 to 100 wt % over the total anode active material of theinnermost anode mixture layer.
 5. The multilayer anode of claim 1,wherein the natural graphite of the outermost anode mixture layer iscontained in an amount of 50 to 100 wt % over the total anode activematerial of the outermost anode mixture layer.
 6. The multilayer anodeof claim 1, wherein the anode mixture layer located between theinnermost anode mixture layer and the outermost anode mixture layercomprises one or more anode active materials selected from artificialgraphite, natural graphite, soft carbon, hard carbon, acetylene carbonblack, Ketjen black, carbon nanotubes, carbon nanofiber and siliconoxide.
 7. The multilayer anode of claim 1, wherein the plurality ofanode mixture layers further comprise a binder and a conductivematerial.
 8. A lithium secondary battery comprising the multilayer anodeaccording to claim
 1. 9. The multilayer anode of claim 1, wherein thecarbon material comprises graphite.
 10. The multilayer anode of claim 1,wherein the carbon material further comprises artificial graphite. 11.The multilayer anode of claim 10, wherein the carbon material furthercomprises natural graphite.